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Issued 19 January 2012The data presented in this drought report are preliminary. Ranks, anomalies, and percent areas may change as more complete data are received and processed.

On a month-by-month basis, 2011 was characterized by large areas of dry and large areas of wet weather. Nine months (all except February, October, and November) had ten percent or more of the country experiencing very dry (at the tenth percentile of the historical record or drier) precipitation anomalies, with two months (August and December) having more than a fifth (20 percent) of the country very dry. This was counterbalanced by large areas (ten percent or more of the country) experiencing very wet (monthly precipitation totals at the 90th percentile of the historical record or wetter) precipitation anomalies in February through May, September, and December. The spring months were especially wet with April and May having more than a fifth of the country very wet. When averaged together, the wet and dry anomalies resulted in the 14th driest January, 24th driest June, 19th driest July, and 29th driest August, nationally, in the 1895-2011 record. Large areas of the country also experienced unusually warm conditions. Ten percent or more of the contiguous U.S. was very warm (monthly temperatures at the 90th percentile of the historical record or warmer) during eight months (all except January, February, May, and October). Conditions were especially warm during the summer, with nearly half (49 percent) of the country very warm in July and over 40 percent very warm in August. The unusual and persistent warmth, especially during the growing season, increased evaporation and intensified local drought conditions. The monthly values for June, July, and August, when averaged together, resulted in 16 percent of the country very dry and 41 percent very warm, or a total of 57 percent very warm and/or dry. This is the second highest such summer total in the 1895-2011 record.

The year started out with areas of drought stretching from the Southwest and Central Plains to the Southeast, with moderate to extreme drought covering about 18 percent of the country (based on the U.S. Drought Monitor [USDM]). By the end of spring, drought had expanded and intensified, especially in the South, with 21 percent of the country in moderate to exceptional drought. The heat and dryness of summer further expanded and intensified drought, with expansion occurring in the Ohio to Mid-Mississippi valleys and about 28 percent of the U.S. classified in the moderate to exceptional drought categories. Some contraction occurred in the drought area during the autumn and early winter months, especially in the Midwest to Central Gulf Coast, but drought developed in the Upper Mississippi Valley, Northern Plains, and Far West, with about 27 percent of the country in moderate to exceptional drought by year's end. The percent area of the U.S. experiencing the worst USDM category (D4, exceptional drought) reached a peak of about 10 percent during the summer and early fall, which is a record in the 12-year history of the USDM. Drought continued during 2011 in Hawaii, but not as severe as it was in 2010.

The percent area* of the contiguous U.S. experiencing moderate to extreme drought (based on the Palmer Drought Index) started the year at about 18 percent, grew steadily to a peak of about 29 percent during the summer, then contracted slightly during the last two months, ending the year at about 25 percent. The Palmer Drought Index data go back 117 years.

*This drought statistic is based on the Palmer Drought Index, a widely used measure of drought. The Palmer Drought Index uses numerical values derived from weather and climate data to classify moisture conditions throughout the contiguous United States and includes drought categories on a scale from mild to moderate, severe and extreme.

The drought epicenters during 2011 were the Southwest to Southern Plains, the Central Gulf Coast to Southeast, and Hawaii. Secondary drought areas developed in the Midwest, Upper Mississippi Valley to Northern Plains, and Far West. Low stream, reservoir and stock pond levels, and depleted soil moisture combined with high temperatures and evaporation to ravage agricultural (pasture, range and crop) lands as the growing season progressed.

Southwest and Southern Plains:

The Southwest (Arizona-New Mexico-Colorado) drought peaked in mid-summer with nearly 40 percent of the region experiencing extreme to exceptional drought.

Drought conditions in Hawaii were not as severe in 2011 as they were in 2010, but moderate to extreme drought lingered with significant long-term rainfall deficits remaining (last 12, 24, 36 months). Severely dry conditions caused by La Niña affected other Pacific islands under the jurisdiction of the United States early in 2011. The dry conditions peaked in February. Kapingamarangi was especially hard hit, with September 2010-February 2011 rainfall totalling 15 percent of normal and water conservation measures strongly encouraged at that time. Above-normal rainfall finally returned to Kapingamarangi in June 2011.

Tree ring records provide a useful paleoclimatic index that extends our historical perspective of droughts centuries beyond the approximately 100-year instrumental record. Several paleoclimatic studies have shown that droughts as severe as, or worse, both in magnitude and duration, than the major 20th century droughts have occurred in the U.S. during the last thousand years. The following paleodrought report was prepared by the NOAA/NCDC Paleoclimatology and Climate Monitoring branches during 2011: